Server 2012 Standard includes two virtual use licenses, while Datacenter includes unlimited and the free Hyper-V Server doesn’t include any
Virtual use licenses are only allowed so long as the host server is not running any other role other than Hyper-V
You can convert from standard to datacenter by entering the following command at the command prompt:
dism /online /set-edition:ServerDatacenter /productkey:48HP8-DN98B-MYWDG-T2DCC-8W83P /AcceptEULA
The key above is a generic MS activation key so after the upgrade is completed enter your MAK or KMS key to activate the server
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